1. Field of the Invention
The present invention relates generally to liquid crystal displays used for displaying information in mobile phones, mobile terminals, and various electric home appliances.
2. Description of the Background Art
Liquid crystal does not emit light itself and controls externally applied light transmitted therethrough for display. Methods of allowing external light to be transmitted through the liquid crystal are divided into two kinds. One is a direct vision or transmissive type, according to which a backlight is provided in the backside of the liquid crystal seen from the viewer and light emitted from the backlight is transmitted through the liquid crystal. The other is a reflective type, according to which light coming in from the front of the liquid crystal is reflected upon a reflecting board provided in the backside of the liquid crystal, and the reflected light is transmitted through the liquid crystal. The reflective type liquid crystal display which does not require a backlight has been vigorously developed, because this type of displays are power saving, thin and lightweight. In addition, since members for the backlight are not necessary, the cost may be reduced.
There have been proposed reflective type liquid crystal displays of various constructions (see, for example, Gekkan LCD Intelligence, April, 1997, pp. 54-58 showing reflective type liquid crystal displays of various constructions). In the reflective type liquid crystal display, light transmitted through a liquid crystal portion should have a scattering property in order to allow sufficient light to reach the viewer from every position of the liquid crystal picture plane. Therefore, the reflective type liquid crystal display has a member to provide the scattering property to the light reaching the viewer. In view of the positions of optical members to provide the scattering property, the constructions of devices may be divided into the following three kinds: (1) the scattering property is provided in the backside of the liquid crystal seen from the viewer, (2) the scattering property is provided in the front of the liquid crystal, and (3) the scattering property is provided within the liquid crystal itself.
FIG. 7 is an illustration of an example of the reflective type liquid crystal display in the case of the above (1). In FIG. 7, the viewer is positioned in the upper part, in other words, in the front, and light comes in from the upper part. A polarizer 1 absorbs a polarized light component orthogonal to one direction (referred to as "S polarized light"), and transmits a polarized light component parallel to that one direction (referred to as "P polarized light"). Liquid crystal 3 is TN (Twisted Nematic) liquid crystal with the most general twist angle of 90.degree.. Transparent substrates 2 and 4 composed of glass, plastic or the like are provided in the front and backside of liquid crystal 3, respectively. A polalizer 5 has an absorbing axis orthogonal to that of polarizer 1 (in the cross Nicol relation). Provided in the backside of polarizer 5 is a reflecting board 6 having a surface of scattering type white resin or a metallic surface with a high reflectance such as silver and aluminum treated to have a scattering property
Polarizer 1 absorbs the S polarized light in the incident light and transmits the P polarized light. When a voltage is not applied to liquid crystal 3, the P polarized light transmitted through TN liquid crystal 3 with the twist angle of 90.degree. is converted into S polarized light, which is then transmitted through polarizer 5. The S polarized light scattered upon reflecting board 6 is once again changed into P polarized light, which is then transmitted through polarized light 1 to reach the viewer, and therefore white display is made.
At this time, color display is also enabled instead of the white display. If a color filter is inserted between the viewer and the reflecting board, color display is enabled. If the reflecting board selectively reflects a particular wavelength, color display is enabled.
When a voltage is applied to liquid crystal 3, the twist of 90.degree. of liquid crystal 3 is untwined, and therefore the incident P polarized light is transmitted as is through liquid crystal 3, and absorbed by polarizer 5. The light is not reflected to reach the viewer and black display is made as a result.
Note that assuming that the absorbing axes of polarizer 1 and 5 are parallel to each other rather than orthogonal, black display is made when a voltage is not applied to the liquid crystal, and white display is made when a voltage is applied. A color operation mode is also permitted.
In the above example, light is transmitted through a polarizer four times altogether before reaching the viewer. An ideal polarizer would not absorb light, but in practice about several to 10% light is absorbed and therefore the number of transmission of light through the polarizer is preferably as small as possible in order to provide bright display.
In order to solve this disadvantage, another type of reflective liquid crystal displays has been recently proposed (see, for example, Japanese Patent Laying-Open No. 10-3078). FIG. 8 shows this proposed reflective type liquid crystal display. This display device is characterized by the use of a polarized-light-separating-plate (reflective type polarizer) 71 which transmits a polarized light component P parallel to one direction in incident light and reflects a polarized light component S orthogonal to that one direction. A scattering (diffusion) plate 61 is provided in the front of polarized-light-separating-plate 71, and a light absorbing plate 8 is provided in the backside. Polarizer 1 absorbs S polarized light and transmits P polarized light.
When a voltage is not applied to liquid crystal 3, the P polarized light transmitted through liquid crystal 3 with the twist of 90.degree. is converted into S polarized light. Subsequently, the S polarized light is scattered by scattering plate 61 and reflected by polarized-light-separating-plate 71. The S polarized light once again scattered by scattering plate 61 is again converted into P polarized light, which is transmitted through polarizer 1 to reach the viewer and therefore white display is made.
When a voltage is applied to liquid crystal 3, the twist of 90.degree. is untwined, P polarized light incident to liquid crystal 3 is transmitted through liquid crystal 3 as is, scattered by scattering plate 61, transmitted through polarized-light-separating-plate 71 and absorbed by light absorbing plate 8. The light which is not reflected and hence not emitted toward the front does not reach the viewer and therefore black display is made.
In the conventional example shown in FIG. 8, since one of the polarizer is not necessary with polarized-light-separating-plate 71 functioning both as a reflecting board and a polarizer, brighter display is made accordingly.
When the reflective type liquid crystal display shown in FIG. 8 is used, scattering plate 61 and polarized-light-separating-plate 71 should be combined in order to provide necessary scattering property. In this case, a part of light is scattered to the backside irrespectively of polarization when a normal scattering plate is used, and therefore the contrast is lowered. Incident light transmitted through scattering plate 61 will not be vertical to polarized-light-separating-plate 71, the polarized light separation performance by polarized-light-separating-plate 71 for separating the S polarized light and P polarized light could be lowered in some cases.
Furthermore, as described above, the conventional reflective type liquid crystal display cannot be used in a dark place where there is no incident light. In order to solve this disadvantage, a half mirror is used for the reflecting board, or an opening is provided at a part of the reflecting board to transmit a part of light, such that the device may be used also as a transmissive type display with a backlight provided in the backside. The reflecting board of this construction is however poor in performance, and the picture plane could be disadvantageously dark.